The present invention relates in general to a communication system for carrying out a hand-over which is requested from a radio base station due to the movement of a mobile station, and more particularly to a packet communication system for managing packet signals and an Asynchronous Transfer Mode (ATM) system for managing cell signals.
In recent years, high reliability has been required for a public communication network for transmitting data relating to voice and images. Nowadays having entered the multimedia era, data is transmitted, for example, utilizing mobile communications. Hence for mobile communications, high reliability has been required when carrying out the hand-over of calls.
A discussion of the hand-over of call is provided, for example, by the reference xe2x80x9cMobile Communication Network, the 12th series of Information Networkxe2x80x9d, by Mitsuo Yokoyama, Shokoudou.
In the light of the foregoing, it is an object of the present invention to provide a communication system which is capable of realizing hand-over of calls in such a way that during the hand-over resulting from the movement of a mobile station, interruption of communication does not occur.
It is another object of the present invention to provide a communication system which is capable of realizing hand-over in such a way that interruption of communication can be prevented from occurring only by adding a trunk function thereto without changing the circuit configuration of the existing communication system.
It is still another object of the present invention to provide a communication system which can be used as an Asynchronous Transfer Mode (ATM) exchange system for managing cell signals as well as a packet exchange system for managing packet signals.
In order to attain the above-mentioned objects, the present invention provides a communication system which includes a plurality of interface circuits through which communication signals are transmitted between a mobile station and a plurality of radio base stations via transmission paths. A switch is provided for assigning the communication signals to the interface circuits in such a way that the communication signals from the interface circuits are distributed in accordance with header information included in each communication signal indicating how the communication signal is to be transmitted from the radio base stations to the mobile station via the transmission paths. A control unit, connected to both the interface circuits and the switch, receives a hand-over instruction, which has been issued from the radio base station in accordance with the movement of the mobile station, from the associated interface circuit. A diversity hand-over processing unit, connected to both the control unit and the switch, receives both a first communication signal received from the radio base station, as a source of hand-over, which has output a hand-over instruction, and a second communication signal received from the radio base station as the destination of hand-over, which will be handed-over, in accordance with the hand-over instruction to output one of the first and second communication signals to the associated interface circuit. The diversity hand-over processing unit also transmits the communication signal which has been received from the mobile station on the reception side for communication via the radio base station to both the interface circuit associated with the radio base station as source of hand-over and the interface circuit associated with the radio base station as destination of hand-over.
The above-mentioned switch can be used in both an Asynchronous Transfer Mode (ATM) switch for managing cell signals and a packet switch for managing packet signals.
The above-mentioned diversity hand-over processing unit includes an instruction processing unit which operates in accordance with an instruction outputted from the control unit, a synchronous selection unit, and a signal duplicating unit.
The asynchronous selection unit receives, via the switch, when a service area is changed from the radio-base station as source of hand-over to the radio base station as destination of hand-over, both the communication signal from the interface circuit associated with the transmission path extending to the radio base station as source of hand-over and the communication signal from another interface circuit associated with the transmission path extending to the radio base station as destination of hand-over, in accordance with an instruction issued from the instruction processing unit. The synchronous selection unit also synchronizes both the two communication signals with each other, and outputs one of the two communication signals which is more excellent in quality to the interface circuit associated with the radio base station which communicates with the mobile station on the reception side for communication.
The signal duplicating unit receives a communication signal from the interface circuit associated with the transmission path extending to the radio base station which communicates with the mobile station on the reception side for communication via the switch, and transmits the communication signal thus received to both the interface circuit associated with the radio base station as a source of hand-over and the interface circuit associated with the radio base station as destination of hand-over. The signal duplicating unit also transmits the communication signal from the interface circuit associated with the radio base station which communicates with the mobile station on the reception-side for communication to the radio base station as destination of hand-over through that interface circuit.
The synchronous selection unit includes a first switch interface circuit for receiving, through an input port, both the communication signal from the radio base station as source of hand-over and the communication signal from the radio base station as destination of hand-over. A first selector is provided for receiving the communication signals to distribute, and in response to the communication signals, distributing these communication signals in accordance with the header information which the respective communication signals have, and a plurality of buffers are provided for temporarily storing both the communication signal from the radio base station as source of hand-over and the communication signal from the radio base station as destination of hand-over which communication signals have been outputted from the first selector. A synchronous circuit synchronizes the communication signals with each other which have been stored temporarily in the plurality of buffers. A selection circuit orders the respective communication signals in accordance with a predetermined algorithm. A header conversion circuit, having a header conversion table, converts the header conversion table, in accordance with the header information which the communication signals have, in such a way that the communication signals are directed from the switch to the mobile station on the reception side for communication through the interface circuit. A second selector selects among the communication signals from the header conversion circuit, and a second switch interface circuit outputs the communication signal from the second selector to the switch through an output port.
The signal duplicating unit includes a third switch interface circuit for receiving through an output port the communication signal output from the mobile station on the reception side for communication from the switch and a first signal processing unit, having a header conversion table, for changing the header conversion table in such a way that the communication signal of interest is directed to the radio base station as source of hand-over. A second signal processing unit, having a header conversion table, is provided for changing the header conversion table in such a way that the communication signal of interest is directed to the radio base station as destination of hand-over and first and second buffers are provided for temporarily storing the respective communication signals from the first and second signal processing units. A selector selects among the communication signals from the first and second buffers, and a fourth switch interface circuit outputs through an output port the communication signal from the selector to the switch.
By adopting the above-mentioned circuit configuration, during the hand-over, the diversity hand-over processing unit receives both the communication signals from the radio base station as source of hand-over and the radio base station as destination of hand-over which radio base stations are associated with the mobile station on the transmission side for communication, and then one of the two communication signals which is more excellent in quality is transmitted from the switch to the mobile station on the reception side for communication. In addition, the diversity hand-over processing unit transmits the communication signal from the mobile station on the reception side for communication to both the radio base station on the origin of hand-over and the radio base station on the destination of hand-over, and in accordance with the completion of the hand-over, the communication is established between the mobile station on the transmission side for communication and the mobile station which will receive the hand-over instruction and which will receive the communication signal. Therefore, during the hand-over, no interruption of the communication occurs at all.